1. Field of the Invention
The present invention relates to a process for the gas phase polymerization of monomeric vinyl chloride (hereinafter referred to as "VCM") or a mixture comprising a major amount of VCM and a minor amount of a comonomer copolymerizable therewith, in which the reactivity is highly improved.
2. Description of the Related Art
It is known from Japanese Examined Patent Publications (Kokoku) No. 48-14666 and No. 52-44918 and U.S. Pat. No. 3,578,646 that the gas phase polymerization of VCM can be carried out in the presence of a radical initiator.
The gas phase polymerization is conducted under an operation pressure Po lower than the saturated vapor pressure Ps of VCM at the polymerization temperature. Namely, in the gas phase polymerization, the relative pressure Pr (Pr=Po/Ps) is maintained within the range of 1&gt;Pr&gt;0.5, and thus it is thought that the reactivity of the gas phase polymerization is lower than the reactivity of the liquid phase polymerization. However, in practice, the reaction speed of the gas phase polymerization is unexpectedly high; for the following reasons:
(1) A seed polymer prepared by preliminary bulk polymerization has a high porosity and the amount of VCM absorbed by the seed polymer is unexpectedly large, although this amount depends on the polymerization temperature and the value Pr. Namely, the amount of VCM absorbed in the seed polymer is 20 to 65% by weight (on a dry basis) and this VCM participates in the above-mentioned reaction.
(2) In the batchwise liquid phase reaction, the amount of initially charged VCM is decreased with the advance of the polymerization, and in the later stage of the polymerization, the amount of VCM to be reacted is rapidly decreased. On the other hand, in the gas phase polymerization, the reaction pressure is kept constant and fresh VCM is supplied to compensate for the VCM absorbed and used for the polymerization. Namely, with the increase of the amount of a powder formed by the polymerization, the total amount of VCM present in the reaction system is increased.
(3) Since there is no continuous phase in the gas phase polymerization, a termination is difficult to occur, as compared with the liquid phase reaction. Accordingly, the apparent propagation rate is enhanced.
(4) In the liquid phase reaction, there is ordinarily adopted a method in which an initiator is first added to initiate the polymerization and the initiator is not additionally supplied midway through the reaction. Ordinarily, the liquid phase reaction is advanced along an S-shaped curve. Namely, at the start the reaction speed is low, then the reaction speed is linearly increased halfway and then further accelerated, and finally, the reaction becomes gentle and is stopped. In short, the reaction rate relative to the polymerization time can be shown by a sharp-peaked curve. Recently, however, efforts have been made to change this sharp-peaked curve to a gentle trapezoidal curve by using an initiator having a low temperature activity in combination with an ordinary initiator. On the other hand, in the gas phase polymerization, if the reaction pressure is maintained constantly by supplying VCM in an amount corresponding to the amount of VCM converted to a polymer, and if the feed rate of VCM is measured, the amount of VCM reacted at any time can be precisely known. Therefore, if the initiator is additionally supplied when the reaction rate is reduced. Accordingly, it is theoretically possible to maintain a linear reaction rate.
As can be seen from the above description, the reactivity of the gas phase polymerization is unexpectedly high even if the reaction is advanced in the gas phase.
Typical instances of initiators used for the gas phase polymerization are shown in the following Table 1. These initiators are appropriately selected according to the polymerization temperature.
TABLE 1 __________________________________________________________________________ Temperature (.degree.C.) at which half Abbreviation Molecular life period is (trade mark) weight 10 hours __________________________________________________________________________ .alpha.-cumylperoxy neodecanate CNDP 306.9 36.6 2,4,4-triethylpentyl-2- TMP-PA 280.4 38.5 peroxyphenoxy acetate diethylperoxy dicarbonate DEP 178.1 39.0 di-3-methoxybutylperoxy MC 294.3 43.0 dicarbonate dimethoxyisopropylperoxy MIP 266.3 43.4 dicarbonate di-2-ethoxyethylperoxy EEP 266.3 43.0 dicarbonate di-3-methyl-3-methoxy- MBP 322 46.8 butylperoxy dicarbonate bis-4-t-butylcyclohexyl- Percadox 16 398.5 46.0 peroxy dicarbonate di-2-ethylhexylperoxy OPP 346.5 47.0 dicarbonate diisopropylperoxy IPP 206.2 47.0 dicarbonate t-butylperoxy neodecanate ND 244.4 48.0 t-butylperoxy pivalate PV 174.2 55 2,2-azobis(2,4-dimethyl- AIBN 248.6 valeronitrile) succinic acid peroxide SA octanoyl peroxide O decanoyl peroxide D lauroyl peroxide LPO 62.0 3,5,5-trimethylhexanoyl 355 314.5 59.5 peroxide benzoyl peroxide BPO __________________________________________________________________________
Another characteristic feature of the gas phase polymerization is that the reactivity depends greatly on the value Pr.
As the value Pr is increased, the reactivity becomes extremely high, the bulk density is increased, the particle size distribution becomes sharper, and the portion of coarse particles is decreased. Moreover, an improvement is found in the initial coloration (hue) at the time of processing of the product and in the thermal stability. However, the polymerization in the interior of particles is advanced, the porosity is decreased and then fish eye characteristic (hereinafter referred to as "FE characteristic") is worsened.
As pointed out hereinbefore, the gas phase polymerization takes place within the range of 0.5&lt;Pr&lt;1. However, in order to maintain the FE characteristic at a good level, and other properties at a practical level, it is preferred that the value Pr be in the range of 0.65&gt;Pr&gt;0.85, especially 0.70 to 0.80. For example, the amount of polyvinyl chloride (hereinafter referred to as "PVC") formed per gram of the net amount of the organic peroxide (initiator: I) for 1 hour at a value Pr of 0.75 at a polymerization temperature providing an average degree of polymerization (hereinafter referred to as "P") of about 1000 is 350 to 450 g-PVC/g-I.hour. This reactivity is higher than the reactivity at a polymerization temperature providing P of about 1000 in the suspension polymerization, which is 250 to 350 g-PVC/g-I.hour. However, in the gas phase polymerization, a relatively larger space volume is necessary in the upper portion of the reaction vessel. Therefore, the productivity per unit volume of the reaction vessel is equal or slightly lower than in the suspension polymerization.